In radar object detection systems, the desired detection space is usually partitioned into numerous adjacent regions of increasing distance from the system antennas. Each of these regions may be referred to as a "range bin", and the system output indicates the presence of an object or target in a particular range bin. In general, the radar system "illuminates" the detection space with high frequency energy with a transmitter antenna, an object in the detection space scatters a portion of the illumination, and the system observes the scattered illumination with a receiver antenna. Some systems observe the scattered illumination from all of the range bins simultaneously, while other systems observe the scattered illumination from only one range bin at a time.
A generally undesirable characteristic of radar systems of the type described above is that the object detection sensitivity is much greater for range bins that are close to the system antennas than for range bins that are further away. The characteristic arises in part because the illumination (and therefore, the scattered energy) is naturally stronger at close range, and in part because side-lobe patterns of the transmitting antenna increase the effective illumination at short range. The side-lobe illumination patterns also have the effect of increasing the detection angle at short range, which is often undesirable in obstacle detection systems. For these reasons, various techniques have been developed for reducing detection sensitivity of the side-lobes, and effectively normalizing the system sensitivity versus range. In pulse-type radar systems, for example, an electronically controlled device may be incorporated into the receiver to attenuate the received signal as a function of time. This technique is referred to as a "sensitivity time control", and is not applicable to continuous wave radar systems since the returned energy from all of the targets in all of the range bins are present at all times. Another approach is to incorporate a side-lobe canceling circuit in the receiver channel for distinguishing between the main-lobe and side-lobe signals by generating an auxiliary signal that closely matches the side-lobe signal. A further approach is to establish detection thresholds which vary among range bins based on distance from the antennas. Each of these approaches involves the addition of circuitry to the system, and therefore increases the system complexity and cost.